Rechargeable batteries are used in many circumstances to power user devices, especially radio communication apparatus such as portable telephones, for example. A charger comprising a voltage generator is provided to recharge the battery. The charger may take the form of an AC/DC converter or a DC/DC transformer, for example.
The charger is often connectable to the user device with the battery connected so that not only can the battery be charged but the device can be used while the battery is charging. A problem arises however if the battery is completely discharged since, when the charger is connected and starts to charge the battery, the supply voltage appearing at the user device is too low for the user device to function until the battery has charged partially and remains too low for an inconveniently long time. This can be particularly troublesome in the case of a portable telephone, for example, if the delay is of the order of minutes when the user desires to make an emergency call.
It is possible to provide control circuits which control the voltage and current supplied by the voltage generator and provide a parallel path for charging the battery. However these control circuits have involved a substantial additional cost.
It is desirable to provide a power supply with control circuits enabling the voltage generator to power the user device even in the presence of a completely discharged battery with a smaller, or no additional cost compared to voltage supplies without such a ‘dead battery capability’.